Measurement of a property of a film, such as film thickness, may be required in a variety of situations, such as measurement of the thickness of an oil film within an engine or machine. This measurement can be used to ensure that a lubricating film is adequately thick, or to measure leakage, in the event that the film is formed by leaking fluid. In other situations, measurement may not be required, but merely detection, such as threshold detection when the thickness passes a predetermined value.
Previous proposals for film thickness measurement have included electrodes spaced apart on a substrate. A film of material formed on the substrate therefore bridges the gap between the electrodes. Voltages applied to the electrodes allow measurements of resistance and capacitance across the gap. These values will change with changes in film thickness. However, previous proposals have been found prone to saturation in their response characteristics as film thickness increases, and have thus been limited in their usefulness.